Drive circuit for light emitting diode array based on a buck-boost topology

ABSTRACT

Features of the invention are related to drive circuits that provide both power factor correction and voltage conversion in a single circuit and in some implementations drive an LED array. This single stage converter uses a buck-boost topology and can operate LED arrays that have DC voltages above or below the AC line voltage. An output side filter and bulk energy storage device is placed at the output of the circuit. Bulk energy storage can be provided by an inductor, a capacitor, or a combination of an inductor and a capacitor. The output side filtering can be provided by a capacitor or a capacitor and an inductor. The circuit also includes a switch and a switch control device for controlling the switch to provide power factor correction.

RELATED APPLICATION

This application is related to U.S. Ser. No. 12/915,685, entitled DriveCircuit for Light Emitting Diode Array Based on SEPIC or CUK Topology,filed Oct. 29, 2010, the entirety of which is incorporated by reference.

FIELD OF THE INVENTION

The present invention is related to power circuits, and moreparticularly to drive circuits, such as those used with light emittingdiodes.

BACKGROUND OF THE INVENTION

Light emitting diodes (LEDs), including LED arrays, are commonly usedfor lighting applications. Some applications are powered by an AC powersource and require multiple circuits to transform the AC input to anapproximately constant current or an approximately constant voltage topower the LEDs. Each of the circuits typically performs a singlefunction. For example, some applications use a power factor correctioncircuit that feeds a bulk energy storage device and a separate powerconversion circuit. The use of separate circuits requires a relativelylarge number of components, which affects the cost of the system. Thebulk energy storage device is typically an electrolytic capacitor. Onedisadvantage of using an electrolytic capacitor in these types ofapplications is that the electrolytic capacitor is relatively expensiveand has a relatively short life. In addition, the electrolytic capacitoris typically the component with the lowest reliability in the ballastand drive circuits. It would reduce the cost and increase thereliability of LED drive circuits if the number of components needed todrive the LEDs could be reduced and if those components could be onesthat are less costly and more reliable.

SUMMARY

Aspects of the invention provide drive circuits that provide both powerfactor correction and voltage conversion in a single stage circuithaving fewer and more reliable components than existing circuits. Thesecircuits place the bulk energy storage device at the output of thesingle stage circuit.

One drive circuit includes a first load connection point and a secondload connection point for connecting a load, such as an LED array, to anoutput of the circuit. The output side filter and bulk energy storagedevice is connected to the first load connection point and to the secondload connection point and to the cathode of a free wheeling diode. Thebulk energy storage device stores energy from one power line cycle tothe next power line cycle. In some circuits the bulk energy storagedevice is an inductor, in other circuits the bulk energy storage deviceis a capacitor, while in yet other circuits the bulk energy storagedevice is a combined device that uses both a capacitor and an inductor.The output side filter provides additional converter frequency ripplefiltering to that provided by the bulk energy storage device. Thosecircuits that use an inductor as the bulk energy storage device alsoinclude a capacitor connected between the first load connection pointand the inductor to provide additional output side filtering. Thosecircuits that use a capacitor as the bulk energy storage device may alsoinclude an inductor connected to the cathode of the free wheeling diodeand to the first load connection point to provide additional output sidefiltering. In those circuits that use a combination of a capacitor andan inductor as the storage device, the capacitor and the inductor alsoprovide output side filtering.

The cathode of the free wheeling diode is connected to the output sidefilter and bulk energy storage device and the anode of the diode isconnected to a switch. In some circuits, the switch is a MOSFET, but anyother suitable switch can be used. A control device controls the switchto provide power factor correction of the input of the drive circuitusing any one of several known control modes, such as a control modewhere the on-time of the switch is inversely proportional to the inputvoltage to the circuit when the converter frequency is held constant.The drive circuit may also include other components, including an inputfilter and another inductor. The drive circuit provides both powerfactor correction and load regulation in a single stage.

The drive circuit may be used with an LED array. The circuit includes afirst LED connection point and a second LED connection point forconnecting the LED array to the output of the circuit. In some of thesecircuits, the bulk energy storage device is an inductor and the outputside filtering is provided by the inductor and an output side capacitor.The inductor is connected to the second LED connection point and to theoutput side capacitor and the cathode of a free wheeling diode. Theoutput side capacitor is connected to the first LED connection point andto the inductor and the cathode of the free wheeling diode. The freewheeling diode is connected via its anode to a switch. The switch mayalso be connected to an optional current sensing resistor. A controldevice controls the switch to provide power factor correction so thatthe circuit provides both power factor correction and voltageconversion.

In another circuit, the circuit includes a first LED connection pointand a second LED connection point for connecting an LED array to anoutput of the circuit and a capacitor as the bulk energy storage device.The capacitor also provides output side filtering. In this circuit,there may also be an optional inductor to provide additional output sidefiltering. If there is no inductor, then the capacitor is connectedbetween the first LED connection point and the second LED connectionpoint. If the inductor is present, then the capacitor is connected tothe first LED connection point and to the inductor and the cathode of afree wheeling diode. The circuit further includes a switch, a controldevice for controlling the switch and an optional current sensingresistor. The circuit provides both power factor correction and loadregulation in a single circuit stage.

In at least one of the circuits, the control device controls the switchbased on inputs corresponding to current detected by a current sensedevice, the current through a current sense resistor, and/or the outputvoltage.

Other features, advantages, and objects of the present invention will beapparent to those skilled in the art with reference to the remainingtext and drawings of this application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a prior art LED driver circuit.

FIG. 2A is a circuit diagram of a prior art power circuit for an LEDdriver circuit driven from a low voltage DC source.

FIG. 2B is a circuit diagram of a prior art power circuit for an LEDdriver circuit driven from an AC source.

FIG. 3 is a block diagram of an exemplary LED driver circuit.

FIG. 4 is a circuit diagram of an exemplary power portion for the LEDdriver circuit of FIG. 3.

FIG. 5 is a circuit diagram of an exemplary power portion for an LEDdriver circuit.

FIG. 6 is a graph illustrating voltage and current at a selected pointin the circuit of FIG. 5.

FIG. 7 is a graph illustrating voltage and current at the LED array forthe circuit of FIG. 5.

FIG. 8 is a circuit diagram of a portion of an exemplary LED drivercircuit.

FIG. 9 is a circuit diagram of an exemplary LED driver circuit with asnubber circuit.

FIG. 10 is a circuit diagram of an exemplary LED driver circuit with asnubber circuit.

FIG. 11 is a circuit diagram of an exemplary LED driver circuit with asnubber circuit.

FIG. 12 is a circuit diagram of an exemplary power portion for the LEDdriver circuit of FIG. 3.

FIG. 13 is a circuit diagram of an exemplary portion of an LED drivercircuit.

FIG. 14 is a graph illustrating voltage and current at a selected pointin the circuit of FIG. 13.

FIG. 15 is a graph illustrating voltage and current at the LED array forthe circuit of FIG. 13.

FIG. 16 is a circuit diagram of an exemplary portion of an LED drivercircuit.

FIG. 17 is a circuit diagram of an exemplary power portion of the LEDdriver circuit of FIG. 3.

FIG. 18 is a circuit diagram of an exemplary portion of an LED drivercircuit.

FIG. 19 is a graph illustrating voltage and current at a selected pointin the circuit of FIG. 18.

FIG. 20 is a graph illustrating voltage and current at the LED array forthe circuit of FIG. 18.

FIG. 21 is a circuit diagram of an exemplary portion of an LED drivercircuit.

DETAILED DESCRIPTION

Circuits implementing the present invention provide a single stagecircuit that provides both power factor correction (PFC) and powerconversion to drive a load, such as an LED array. In some circuits, theoutput of the circuit provides approximately constant current for arange of input voltages, while in other circuits, the output of thecircuit provides approximately constant voltage. The circuit uses anenergy storage device at its output to provide PFC. One advantage ofembodiments of the invention over the prior art is that they can beimplemented using a circuit with fewer components where the componentsallow for lower cost, higher reliability and longer life. The circuitcan utilize a buck-boost, SEPIC or Cuk topology.

FIG. 1 illustrates a block diagram for a prior art system that drives anLED array. The input of the system connects to a AC voltage source (notshown). Block 102 provides EMI filtering and surge protection. Theoutput of block 102 is connected to block 104, which performs powerfactor correction (PFC). FIG. 1 illustrates a boost type PFC circuit.There are a number of commercially available control ICs that providePFC control that can be used in block 104. The output of the PFC circuit104 is connected to a bulk energy storage device 106. The bulk energystorage device is typically an electrolytic capacitor. The bulk energystorage device is connected to a DC-to-DC converter 108. The DC-to-DCconverter provides current or voltage regulation at the load, which inFIG. 1 is an LED array 110. Block 108 also provides isolation if needed.Block 108 is typically a buck-type derivative or a fly-back typecircuit. It may also be a resonant converter with a rectified output.Prior art systems, such as the one illustrated by FIG. 1, typicallyrequire multiple switching devices. For example one switch may beassociated with the power factor correction function in block 104 andone or more switching devices may be associated with the DC-to-DCconverter of block 108.

FIG. 2A illustrates a prior art buck-boost circuit that can be used toprovide an approximately constant voltage to an LED array. This circuitcan operate from a DC power source or an LED driver that provides aconstant voltage. The circuit shown in FIG. 2A does not include PFCcontrol or bulk energy storage. If the DC power source is connected toan AC power line, then the PFC control and bulk energy storage areprovided by the power supply. In this case, a first stage that providesAC to DC conversion would be required, but this first stage is not shownin FIG. 2A.

FIG. 2B illustrates a prior art circuit that includes a first stage thatprovides AC to DC conversion and PFC control. The bridge rectifierprovides the AC to DC conversion and the portion of the circuit indotted lines provides PFC control. FIG. 2B illustrates a boost type PFCcircuit followed by a second buck or buck/boost converter. The largecapacitor, C1, provides bulk energy storage.

Single Stage for PFC and Conversion

FIG. 3 illustrates a block diagram of an exemplary operating environmentfor the invention. The input voltage to the system is typically between100V and 277V AC, which allows the system to work with most U.S. andEuropean AC line voltages. Block 302 provides EMI filtering and surgeprotection. The bridge rectifier 304 rectifies the power signal fromblock 302. The output of the bridge feeds the PFC/converter circuit 306,which provides both PFC and voltage conversion. The bulk energy storagedevice and output side filter 308 are at the output of the PFC/convertercircuit 306. The bulk energy storage device and output side filter helpsprovide a regulated current or a regulated voltage to the load 310. Thebulk energy storage device stores energy from one power line cycle tothe next power line cycle. The output side filtering provides high andlow frequency filtering and stores energy from one conversion cycle tothe next conversion cycle. The exemplary load shown in FIG. 3 is an LEDarray. A comparison of FIG. 1 to FIG. 3 shows that one aspect of theinvention combines the separate PFC 104 and converter 108 circuits intoa single stage circuit, the PFC/converter 306, which provides both PFCand conversion, and moves the bulk energy storage device to the outputof the circuit in order to provide load regulation.

Single Stage with Bulk Energy Storage and Output Side Filtering—Based ona Buck-Boost Topology

FIG. 4 illustrates an exemplary circuit for the PFC/converter 306 andthe bulk energy storage and output side filter 308, based on abuck-boost topology, which allows the circuit to operate LED arrays atvoltages above or below the AC line voltage. Other circuits that providea DC voltage at the LED array of 1-1.2 times the RMS of the lowest ACline voltage could be based on a buck topology. FIG. 4 also includes abridge rectifier 401.

The output side filter and bulk energy storage device 418 is connectedto the LED array 420 at LED connection points 422, 424. As discussed inmore detail in connection with FIGS. 5 and 8, in some circuits the bulkenergy storage device is an inductor and in other circuits the bulkenergy storage device is a capacitor. It is also possible to use acombination of an inductor and a capacitor as the bulk energy storagedevices. Placing the bulk energy storage device at the output of thecircuit eliminates the need for a separate PFC circuit. A capacitor andif present, an inductor provide additional output side filtering. FIG. 4illustrates that the output side filter and bulk energy storage device418 is connected to the first LED connection point 424, the second LEDconnection point 422 and to the cathode of a free wheeling diode 414.Some circuits include an optional current sense device. If the currentsense device, CT 416, is present, then it may be connected between theoutput side filter and bulk energy storage device 418 and the cathode ofthe diode, as shown in FIG. 4.

The circuit includes a switch, Q1 406, controlled by a control device408. The control device controls the switch to provide both PFC and aregulated current or voltage to the LED array, which is connected to theoutput of the circuit at LED connection points 422, 424. In this manner,only a single switch is required to provide both PFC and to control thecurrent or voltage to the LED array. The control device 408 controls theswitch to provide PFC using any one of several known control methods.For example, the on-time of the switch may be proportional to thecurrent provided to the LED array or inversely proportional to the linevoltage. Although FIG. 4 illustrates the switch as a MOSFET, other typesof switches may be used. The control device can be an analog circuit, anASIC, a microprocessor or any other suitable control device.

If the circuit is providing regulated current to the LED array, then insome circuits the control device controls the on-time of the switchbased on inputs, CT-1, CT-2, from an optional current sense device, suchas a current sense transformer, CT 416. In FIG. 4 the current sensedevice senses the current at a point between the cathode of the diode D1414 and the output side filter and energy storage device 418. In othercircuits, the control device controls the on-time of the switch based oninputs from an optional current sensing resistor R_(Sense) 410. In FIG.4 where the switch Q1 410 is implemented with a MOSFET, the currentsensing resistor is connected to the source of the MOSFET. If thecircuit is providing approximately constant voltage to the LED array,then the control device controls the on-time of the switch based onvoltage inputs which represent the voltage across the LED array, V_(LED)to V_(LEDCommon). Some circuits can include both types of inputs to thecontrol device so that the circuit can be used with a variety of LEDarrays, whereas other circuits may include only one type of input to thecontrol device and be used with one type of LED arrays. The controldevice may also use the current I_(Sense) sensed through the currentsensing resistor, R_(Sense) 410, to control the switch on a cycle bycycle basis. If the current sensed through the current sensing resistorindicates an overcurrent condition, then the control device canimmediately open the switch. As will be apparent to those skilled in theart, there are a number of other ways to control the switch to providePFC.

An inductor, L1 412, separate from any inductor in the output sidefilter and bulk energy storage device, is connected to the switch, Q1406 and to the anode of the diode D1 414 and to V_(LEDCommon). The valueof L1 412 depends upon the type of control, as well as the frequency ofoperation and the power requirements of the LED array. For example, inone circuit that operates in a discontinuous conduction mode (DCM), theinductor, L1 412, is relatively small with an exemplary value of 100μH-1 MH. In other circuits that operate in a continuous conduction mode(CCM), the inductor, L1 412, is larger with an exemplary value of 1 MHand above. In FIG. 4 where the switch Q1 410 is implemented with aMOSFET, the inductor, L1 412, is connected to the drain of the MOSFET.The anode of a free-wheeling diode D1 414 is also connected to theswitch Q1 406. There may be a high frequency filter, C1 402, at theinput of the PFC/converter circuit. The high frequency filter may be arelatively small capacitor, C1 402, typically less than 1 μF. In FIG. 4,the high frequency filter is a capacitor, C1 402, and is connected tothe inductor, L1 412, and V_(LEDCommon) and to a circuit input.

Although not shown in FIG. 4, some circuits include lossless orsemi-resonant snubber circuits at the switch to decrease switchinglosses, which in turn improves the reliability of the circuit.Additional details about the inclusion of an optional snubber circuitare discussed below in the section entitled Snubber Circuits.

Example Using an Inductor for Bulk Energy Storage in a Circuit Based ona Buck-Boost Topology

FIG. 5 illustrates one drive circuit based on a buck-boost topologywhere the output side filter and bulk energy storage device 418 aincludes a small output side capacitor, C2 504, and an inductor, L2 502,and the inductor acts as the bulk energy storage device. In thiscircuit, the bulk energy storage device L2 502 is connected to thesecond LED connection point 422 on one side and to the output sidecapacitor, C2 504 and the cathode of the free wheeling diode D1, on theother side. The output side capacitor, C2 504, provides high frequencyfiltering and typically has a value of less than a few μF. The inductor,L2 502, provides low frequency filtering in addition to bulk energystorage and typically has a value between 25 mH to a few Henrys.

The remaining components are similar to those described in connectionwith FIG. 4. There is a high frequency filter at the input, such as acapacitor, C1 402 with a value typically less than a few μF. A controldevice 408 controls the switch, Q1 406, to provide both PFC and toregulate the current or voltage to the LED array. The control of theswitch is similar to that described above in connection with FIG. 4.

FIG. 5 illustrates a circuit that includes the optional current sensedevice 416. The current sense device is useful if the circuit providesan approximately constant current to the LED array 420. If the circuitis providing an approximately constant voltage to the LED array 420,then the control device 408 may control the switch Q1 406 based onvoltage inputs which represent the voltage across the LED array, V_(LED)to V_(LEDCommon). The control device may also use an optional current,I_(Sense), sensed through the current sensing resistor, R_(Sense) 410 toprotect the switch against over current on a cycle by cycle basis.

A second inductor, L1 412, is connected between the high frequency inputfilter, C1, and the switch, Q1 406. The anode of a free-wheeling diodeD1 414 is connected to the switch, Q1, and to one end of the inductor L1412. The cathode of the free wheeling diode D1 414 is connected to theinductor L2 502, and the capacitor, C2 504, through the current sensedevice CT 416 if the circuit includes a current sense device. In thosecircuits that do not use a current sense device, the cathode of the freewheeling diode is connected directly to the inductor L2 502 and thecapacitor, C2 504. The circuit illustrated by FIG. 5 has an output sidefilter and bulk energy storage device at its output, C2 504 and L2 502,and requires only a single switch, Q1 406, to provide both PFC and loadregulation.

FIGS. 6 and 7 illustrate the operation of a circuit, such as thatillustrated in FIG. 5. FIG. 6 illustrates the change in voltageV_(INPUT) and current I_(INPUT) over time at a point between the EMI andsurge protection block and the bridge rectifier. FIG. 7 illustrates theregulated current I_(LED) and the regulated voltage V_(LED) at the LEDarray. The values shown in FIGS. 6 and 7 are exemplary. As will beapparent to one skilled in the art, the values will vary for differentimplementations.

Example Using an Inductor and a Capacitor for Bulk Energy Storage in aCircuit Using a Buck-Boost Topology

FIG. 5 also can be used to illustrate a circuit where the bulk energystorage device is a combined device that uses both a capacitor and aninductor for bulk energy storage. The capacitor and inductor that formthe bulk energy storage device also provide the output side filtering.The difference between this circuit and the circuit discussed abovewhere the bulk energy storage is provided by an inductor is that thevalues of the capacitor and the inductor differ. In this circuit, therelative energy stored and transferred to the load during an AC linecycle is similar (i.e., within one order of magnitude). Thus, bothdevices share in both the AC line frequency energy storage and the highfrequency (converter switching frequency) filtering in a similar manner.For this circuit, the value of the capacitor C2 would typically be inthe 10's of μF and the value of the inductor L2 would typically be inthe 100's of mH.

Example Using a Capacitor as Bulk Energy Storage Device in a CircuitUsing a Buck-Boost Topology

FIG. 8 illustrates an exemplary PFC/converter circuit based on abuck-boost topology where the output side filter and energy storagedevice 418 b includes a capacitor, C2 1102 as the bulk energy storagedevice. In addition to bulk energy storage, the capacitor C2 1102 alsoprovides high and low frequency output side filtering. The output sidefilter and energy storage device 418 b may also include an optionalinductor, L2 1104. If the inductor L2 1104 is included, then it providesadditional output side high frequency filtering.

If the output side filter and energy storage device 418 b includes onlya capacitor, such as C2 1102, then the capacitor is connected to thefirst LED connection point 424 and the second LED connection point 422.If the output side filter and energy storage device 418 b includes botha capacitor C2 1102 and an inductor L2 1104, then the capacitor isconnected to the first LED connection point and to the inductor and theinductor is connected to the second LED connection point and to thecapacitor.

The remaining components are similar to those described in connectionwith FIGS. 4 and 5. A control device 408 controls the switch, Q1 406, toprovide both PFC and an approximately constant current (or voltage) tothe LED array 420. The control of the switch is similar to thatdescribed above in connection with FIGS. 4 and 5. FIG. 8 illustrates anoptional current sense device 416, which is used if the circuit isproviding an approximately constant current to the LED array. There is ahigh frequency filter at the input, such as a small capacitor, C1 402.

An inductor L1 412 is connected between the high frequency input filter,C1 402 and the switch, Q1 406. The anode of a free-wheeling diode D1 416is connected to the switch, Q1, and to one end of the inductor L1. Thecathode of the free wheeling diode D1 is connected to the bulk energystorage device, C2 through the current sense device, if it is present.In those circuits that do not use a current sense device, the cathode ofthe free wheeling diode is connected directly to the capacitor, C2. Thecathode of the free wheeling diode D1 may also be connected to theinductor, L2 if present, either through the current sense device ordirectly to the inductor L2. As will be apparent to those of skill inthe art, other means of current sensing could be used in the circuitillustrated in FIG. 8.

Snubber Circuits

As discussed above, adding a snubber circuit at the switch decreasesswitching losses and improves the reliability of the circuit. FIGS. 9-11provide examples of some circuits that include snubber circuits. FIG. 9illustrates the circuit of FIG. 4 with the addition of a losslesssnubber circuit. Adding a snubber circuit to the circuit of FIG. 4 addsthree diodes, DS1 1202, DS2 1204, DS3 1206, three capacitors, CS1 1208,CS2 1210, C_(Snubber) 1212, and two inductors, LS 1214 and L_(Snubber)1216. FIG. 10 illustrates the circuit of FIG. 5 with the addition of alossless snubber circuit. Adding a snubber circuit to the circuit ofFIG. 5 adds three diodes, DS1 1302, DS2 1304, DS3 1306, threecapacitors, CS1 1308, CS2 1310, C_(Snubber) 1312, and two inductors, LS1314 and L_(Snubber) 1316. FIG. 11 illustrates the circuit of FIG. 5with the addition of a standard R-C snubber circuit. Adding a snubbercircuit to the circuit of FIG. 5 adds a resistor, Rsnubber 1402, and acapacitor, C_(Snubber) 1404. As will be apparent to one of skill in theart, there are many other snubber circuit designs that can be used andthe invention is not limited to those illustrated by the figures.

Additional Circuit Topologies

In addition to the buck-boost topology described above, other circuittopologies can be used for providing PFC and conversion in a singlestage with a bulk energy storage device at the output. The followingsections provide additional details on circuits that use a SEPIC(Single-Ended Primary-Inductor Converter) or Cuk topology.

Single Stage with Bulk Energy Storage and Output Side Filtering—Based ona SEPIC Topology

FIG. 12 illustrates an exemplary PFC/converter 306 and the bulk energystorage and output side filter 308, based on a SEPIC topology, whichallows the circuit to operate LED arrays at voltages above or below theAC line voltage. FIG. 12 also includes a bridge rectifier 1501.

The output side filter and bulk energy storage device 1518 is connectedto a first LED connection point 1524 and a second LED connection point1522, as well as to the cathode of a free wheeling diode, D1 1514. Asdiscussed in more detail in connection with FIGS. 13 and 16, in somecircuits the bulk energy storage device is a capacitor and in othercircuits the bulk energy storage device is an inductor. It is alsopossible to use a combination of an inductor and a capacitor as the bulkenergy storage devices. Placing the bulk energy storage device at theoutput of the circuit eliminates the need for a separate PFC circuit. Acapacitor and if present, an inductor provide additional output sidefiltering. FIG. 12 illustrates an inductor, L2 1528, is connected to theanode of the diode, D1 1514, and to the first LED connection point 1524.A capacitor, C2 1526, is connected at one end to both the anode of thediode, D1 1514, and the inductor, L2 1528, and at the other end toanother inductor, L1 1512. An optional resistor, R_(LED) 1530 is shownin FIG. 12, which is connected to the first LED connection point 1524and is in series with the LED array 1520. The resistor, R_(LED) 1530provide LED current sensing.

The circuit includes a switch, Q1 1506, controlled by a control device1508. The control device controls the switch to provide both PFC and aregulated current or voltage to the LED array, which is connected to theoutput of the circuit at LED connection points 1522, 1524. In thismanner, only a single switch is required to provide both PFC and tocontrol the current or voltage to the LED array. The control device 1508controls the switch to provide PFC using any one of several knowncontrol methods. For example, the on-time of the switch may beproportional to the current provided to the LED array or inverselyproportional to the line voltage. Although FIG. 12 illustrates theswitch as a MOSFET, other types of switches may be used. The controldevice can be an analog circuit, an ASIC, a microprocessor or any othersuitable control device.

If the circuit is providing regulated current to the LED array, then insome circuits the control device controls the on-time of the switchbased on inputs, CT-1, CT-2, from an optional current sense device, suchas a current sense transformer, CT 1516. In FIG. 12 the current sensedevice senses the current through the inductor, L1 1512. In othercircuits, the control device controls the on-time of the switch based oninputs from an optional current sensing resistor R_(Sense) 1510. In FIG.12 where the switch Q1 1510 is implemented with a MOSFET, the currentsensing resistor is connected to the source of the MOSFET. If thecircuit is providing approximately constant voltage to the LED array,then the control device controls the on-time of the switch based onvoltage inputs which represent the voltage across the LED array, V_(LED)to V_(outcom). Some circuits can include both types of inputs to thecontrol device so that the circuit can be used with a variety of LEDarrays, whereas other circuits may include only one type of input to thecontrol device and be used with one type of LED arrays. The controldevice may also use the current I_(Sense) sensed through the currentsensing resistor, R_(Sense) 410, to control the switch on a cycle bycycle basis. If the current sensed through the current sensing resistorindicates an overcurrent condition, then the control device canimmediately open the switch. As will be apparent to those skilled in theart, there are a number of other ways to control the switch to providePFC.

The inductor, L1 1512, is connected to the switch, Q1 406, and to C21526. The value of L1 1512 depends upon the type of control, as well asthe frequency of operation and the power requirements of the LED array.For example, in one circuit that operates in a discontinuous conductionmode (DCM), the inductor, L1 1512, is relatively small with an exemplaryvalue of 100 μH-1 MH. In another circuit that operates in a continuousconduction mode (CCM), the inductor, L1 1512, is larger with anexemplary value of 1 MH and above. In FIG. 15 where the switch, Q1 1510,is implemented with a MOSFET, the inductor, L1 1512, is connected to thedrain of the MOSFET. There may be a high frequency filter, C1 1502, atthe input of the PFC/converter circuit. The high frequency filter may bea relatively small capacitor, C1 1502, typically less than 1 μF.

FIG. 13 illustrates an exemplary PFC/converter circuit based on a SEPICtopology where the output side filter and energy storage device 1518 aincludes a capacitor, C3 1602, as the bulk energy storage device. Inaddition to bulk energy storage, the capacitor C3 1602 also provideshigh and low frequency output side filtering. In one implementation, thebulk energy storage device includes four 100 μF capacitors or theequivalent thereof. If the output side filter and energy storage device1518 a includes only a capacitor, such as C3 1602, then one side of thecapacitor is connected to the LED connection point 1522 and the cathodeof the diode 1526 and the other side of the capacitor is connected tothe other LED connection point 1524.

The remaining components are similar to those described in connectionwith FIG. 12. A control device 1508 controls the switch, Q1 1506, toprovide both PFC and an approximately constant current (or voltage) tothe LED array 1520. The control of the switch is similar to thatdescribed above in connection with FIG. 12. FIG. 13 illustrates anoptional current sense device 1516, which is used if the circuit isproviding an approximately constant current to the LED array. There is ahigh frequency filter at the input, such as a small capacitor, C1 1502.

An inductor, L1 1512, is connected between the high frequency inputfilter, C1 1502, and the switch, Q1 1506, as well as to C2 1526. Thecapacitor, C2 1526, is connected on one side to the inductor, L1 1512,and the switch, Q1 1506, and on the other side to another inductor, L21528, and the anode of the diode, D1 1514.

FIGS. 14 and 15 illustrate the operation of a circuit, such as thatillustrated in FIG. 13. FIG. 14 illustrates the change in voltageV_(INPUT) and current I_(INPUT) over time at a point between the EMI andsurge protection block and the bridge rectifier. FIG. 15 illustrates theregulated current I_(LED) and the regulated voltage V_(LED) at the LEDarray. The values shown in FIGS. 14 and 15 are exemplary. As will beapparent to one skilled in the art, the values will vary for differentimplementations.

Although FIG. 13 illustrates a circuit where the output side filter andbulk energy storage device is a capacitor, similar to the buck-boostcircuits described above, in other circuits the output side filter andbulk energy storage device may include both a capacitor and an inductorand the bulk energy storage may be provided by the inductor or acombination of a capacitor and an inductor.

FIG. 16 illustrates an example where the output side filter and bulkenergy storage device includes both a capacitor, C3 1602, and aninductor, L3 1604. In one implementation, the capacitor, C3 1602, actsas the bulk energy storage device and provides output side filtering andthe inductor, L3 1604, provides additional output side filtering. In asecond implementation, both the capacitor, C3 1602, and the inductor, L31604, provide bulk energy storage, as well as output side filtering. Ina third implementation, the inductor, L3 1604 provides bulk energystorage and output side filtering and the capacitor, C3 1602 providesadditional output side filtering. In FIG. 16, the inductor, L3 1604 isconnected to the cathode of the diode, D1 1514 and the capacitor, C31602, and to the second LED connection point 1522. The remainingcomponents are similar to those described in connection with FIG. 14.

Single Stage with Bulk Energy Storage and Output Side Filtering Based ona Cuk Topology

FIG. 17 illustrates an exemplary PFC/converter 306 and bulk energystorage and output side filter 308, based on a Cuk topology, whichallows the circuit to operate LED arrays at voltages above or below theAC line voltage. FIG. 17 also includes a bridge rectifier 2101. Acomparison between FIGS. 12 and 17 shows that both the SEPIC and Cuktopologies include a sub-circuit that includes a capacitor, twoinductors, a switch and a diode connected between the input of thecircuit and the output side filter and bulk energy storage device, butthat the arrangement of the capacitor, inductors, switch and diodediffer between the two topologies.

The output side filter and bulk energy storage device 2118 is connectedto the first LED connection point 2124 and the second LED connectionpoint 2122, as well as an inductor, L2 2128. As discussed in more detailin connection with FIGS. 17 and 21, in some circuits the bulk energystorage device is a capacitor and in other circuits the bulk energystorage device is an inductor. It is also possible to use a combinationof an inductor and a capacitor as the bulk energy storage devices.Placing the bulk energy storage device at the output of the circuiteliminates the need for a separate PFC circuit. A capacitor and ifpresent, an inductor provide additional output side filtering. FIG. 17illustrates that the inductor, L2 2128, is also connected to the anodeof diode, D1 2114, and a capacitor, C2 2126. The capacitor, C2 2126, isalso connected to another inductor, L1, 2112, and the switch, Q1 2106.An optional resistor, R_(LED) 2130 is shown in FIG. 21, which isconnected to the first LED connection point 1524 and is in series withthe LED array 2120. This optional resistor senses the LED current.

The switch, Q1 2106, is controlled by a control device 2108. The controldevice controls the switch to provide both PFC and a regulated currentor voltage to the LED array, which is connected to the output of thecircuit at LED connection points 2122, 2124. In this manner, only asingle switch is required to provide both PFC and to control the currentor voltage to the LED array. The control device 2108 controls the switchto provide PFC using any one of several known control methods. Forexample, the on-time of the switch may be proportional to the currentprovided to the LED array or inversely proportional to the line voltage.In circuits that use a Cuk topology, VLED and ILED are negative and needto be inverted by the control device. Although FIG. 17 illustrates theswitch as a MOSFET, other types of switches may be used. The controldevice can be an analog circuit, an ASIC, a microprocessor or any othersuitable control device.

If the circuit is providing regulated current to the LED array, then insome circuits the control device controls the on-time of the switchbased on inputs, CT-1, CT-2, from an optional current sense device, suchas a current sense transformer, CT 2116. In FIG. 17 the current sensedevice senses the current through the inductor, L1 2112. In othercircuits, the control device controls the on-time of the switch based oninputs from an optional current sensing resistor R_(Sense) 2110. In FIG.17 where the switch, Q1 2110, is implemented with a MOSFET, the currentsensing resistor is connected to the source of the MOSFET. If thecircuit is providing approximately constant voltage to the LED array,then the control device controls the on-time of the switch based onvoltage inputs which represent the voltage across the LED array, V_(LED)to V_(OutCom). Some circuits can include both types of inputs to thecontrol device so that the circuit can be used with a variety of LEDarrays, whereas other circuits may include only one type of input to thecontrol device and be used with one type of LED arrays. The controldevice may also use the current I_(Sense) sensed through the currentsensing resistor, R_(Sense) 2110, to control the switch on a cycle bycycle basis. If the current sensed through the current sensing resistorindicates an overcurrent condition, then the control device canimmediately open the switch. As will be apparent to those skilled in theart, there are a number of other ways to control the switch to providePFC.

The inductor, L1 2112, is connected to the switch, Q1 2106 and to C22126 on one side and to C1 2102 on the other side. The value of L1 2112depends upon the type of control, as well as the frequency of operationand the power requirements of the LED array. For example, in one circuitthat operates in a discontinuous conduction mode (DCM), the inductor, L11512, is relatively small with an exemplary value of 100 μH-1 MH. Inanother circuit that operates in a continuous conduction mode (CCM), theinductor, L1 2112, is larger with an exemplary value of 1 MH and above.In FIG. 21 where the switch Q1 2110 is implemented with a MOSFET, theinductor, L1 2112, is connected to the drain of the MOSFET. There may bea high frequency filter, C1 2102, at the input of the PFC/convertercircuit. The high frequency filter may be a relatively small capacitor,C1 2102, typically less than 1 μF.

FIG. 18 illustrates an exemplary PFC/converter circuit based on a Cuktopology where the output side filter and energy storage device 2118 aincludes a capacitor, C3 2202, as the bulk energy storage device. Inaddition to bulk energy storage, the capacitor, C3 2202, also provideshigh and low frequency output side filtering. Exemplary values for C3range from 300 to 400 μF. If the output side filter and energy storagedevice 2118 a includes only a capacitor, such as C3 2202, then one sideof the capacitor is connected to the LED connection point 2122 and theinductor L2 2128 and the other side of the capacitor is connected to theother LED connection point 2124.

The remaining components are similar to those described in connectionwith FIG. 17. A control device 2108 controls the switch, Q1 2106, toprovide both PFC and an approximately constant current (or voltage) tothe LED array 2120. The control of the switch is similar to thatdescribed above in connection with FIG. 21. FIG. 22 illustrates anoptional current sense device 2116, which is used if the circuit isproviding an approximately constant current to the LED array. There is ahigh frequency filter at the input, such as a small capacitor, C1 2102.

An inductor, L1 2112, is connected between the high frequency inputfilter, C1 2102, and the switch, Q1 2106, as well as to C_(Cuk) 2126.The capacitor, C_(Cuk) 2126, is connected on one side to the inductor,L1 2112, and the switch, Q1 2106, and on the other side to anotherinductor, L_(Cuk) 2128, and the anode of the diode, D1 2114.

FIGS. 19 and 20 illustrate the operation of a circuit, such as thatillustrated in FIG. 18. FIG. 19 illustrates the change in voltageV_(INPUT) and current I_(INPUT) over time at a point between the EMI andsurge protection block and the bridge rectifier. FIG. 20 illustrates theregulated current I_(LED) and the regulated voltage V_(LED) at the LEDarray. The values shown in FIGS. 19 and 20 are exemplary. As will beapparent to one skilled in the art, the values will vary for differentimplementations.

Although FIG. 18 illustrates a circuit where the output side filter andbulk energy storage device is a capacitor, similar to the buck-boostcircuits described above, in other circuits the output side filter andbulk energy storage device may include both a capacitor and an inductorand the bulk energy storage may be provided by the inductor or acombination of a capacitor and an inductor.

FIG. 21 illustrates an example where the output side filter and bulkenergy storage device includes both a capacitor, C3 2202, and aninductor, L3 2204. In one implementation, the capacitor, C3 2202, actsas the bulk energy storage device and provides output side filtering andthe inductor, L3 2204, provides additional output side filtering. In asecond implementation, both the capacitor, C3 2202, and the inductor, L32204, provide bulk energy storage, as well as output side filtering. Ina third implementation, the inductor, L3 2204 provides bulk energystorage and output side filtering and the capacitor, C3 2202 providesadditional output side filtering. In FIG. 21, the inductor, L3 2204 isconnected to inductor L2 2128 and C3 2202 on one side and to the secondLED connection point 2122 on the other side. The remaining componentsare similar to those described in connection with FIG. 18.

The foregoing description of the exemplary embodiments of the inventionhas been presented only for the purposes of illustration and descriptionand is not intended to be exhaustive or to limit the invention to theprecise forms disclosed. Many modifications and variations are possiblein light of the above teaching. The embodiments were chosen anddescribed in order to explain the principles of the invention and theirpractical application to enable others skilled in the art to utilize theinvention and various embodiments and with various modifications as aresuited to the particular use contemplated. Alternative embodiments willbecome apparent to those skilled in the art to which the presentinvention pertains without departing from its spirit and scope. Forexample, although the embodiments described herein illustrate an LEDarray as the load, the circuit can be used with other types of loadsthat have similar power requirements.

I claim:
 1. A drive circuit, comprising: a first load connection pointand a second load connection point for connecting a load to an output ofthe drive circuit; an output side filter and bulk energy storage device,wherein a first connection point of the output side filter and bulkenergy storage device is connected to the first load connection point, asecond connection point of the output side filter and bulk energystorage device is connected to the second load connection point, and athird connection point of the output side filter and bulk energy storagedevice is connected to a cathode of a free wheeling diode; a firstinductor connected to an anode of the free wheeling diode and a switchand to the first load connection point and the first connection point ofthe output side filter and bulk energy storage device; the free wheelingdiode, wherein the cathode of the free wheeling diode is connected tothe third connection point of the output side filter and bulk energystorage device and the anode of the free wheeling diode is connected tothe first inductor and the switch; the switch connected to the anode ofthe free wheeling diode and the first inductor and to a first drivecircuit input; and a control device for controlling the switch toprovide power factor correction, wherein the drive circuit provides bothpower factor correction and load regulation using the switch, whereinthe drive circuit is operable for direct connection to an output of abridge rectifier, wherein the first drive circuit input connects a firstoutput terminal of the bridge rectifier to the switch and a second drivecircuit input connects a second output terminal of the bridge rectifierto the first load connection point, the first connection point of theoutput side filter and bulk energy storage device, and the firstinductor, wherein the output side filter and energy storage deviceincludes a second inductor and a capacitor, wherein the second inductoris connected to the second load connection point and to the cathode ofthe free wheeling diode and the capacitor, and the capacitor isconnected to the first load connection point and the first inductor andto the second inductor and the cathode of the free wheeling diode, andwherein the capacitor and/or the second inductor provide bulk energystorage by storing energy from one power line cycle to a next power linecycle and both the capacitor and the second inductor provide output sidefiltering.
 2. The drive circuit of claim 1, further comprising: a secondcapacitor, wherein the second capacitor is connected to the first loadconnection point and the first inductor and to the circuit input andwherein the first inductor is connected to the second capacitor and thefirst load connection point and to the switch and the anode of the freewheeling diode.
 3. The drive circuit of claim 1, wherein the circuitprovides a regulated current or a regulated voltage to an LED arrayconnected to the first connection point and the second connection point.4. An LED drive circuit, comprising: a first LED connection point and asecond LED connection point for connecting an LED array to an output ofthe drive circuit; a first inductor connected to the first LEDconnection point and an output side filter and bulk energy storagedevice and to an anode of a free wheeling diode and a switch; the outputside filter and bulk energy storage device, wherein a first connectionpoint of the output side filter and bulk energy storage device isconnected to the first LED connection point and the first inductor, asecond connection point of the output side filter and bulk energystorage device is connected to the second LED connection point, and athird connection point of the output side filter and bulk energy storagedevice is connected to a cathode of the free wheeling diode, wherein theoutput side filter and bulk energy storage device includes a secondinductor that provides bulk energy storage by storing energy from onepower line cycle to a next power line cycle and a capacitor, wherein thecapacitor is connected to the first LED connection point and the firstinductor and to the second inductor and the cathode of the free wheelingdiode, and the second inductor is connected to the second LED connectionpoint and to the capacitor and the cathode of the free wheeling diode,and wherein the second inductor and the capacitor provide output sidefiltering; the free wheeling diode, wherein the cathode of the freewheeling diode is connected to the third connection point of the outputside filter and bulk energy storage device and the anode of the freewheeling diode is connected to the first inductor and the switch; theswitch connected to the anode of the free wheeling diode and the firstinductor and to a first drive circuit input; and a control device forcontrolling the switch to provide power factor correction; wherein thecircuit provides both power factor correction and load regulation usingthe switch, and wherein the drive circuit is operable for directconnection to an output of a bridge rectifier, wherein the first drivecircuit input connects a first output terminal of the bridge rectifierto the switch and a second drive circuit input connects a second outputterminal of the bridge rectifier to the first load connection point, thefirst connection point of the output side filter and bulk energy storagedevice, and the first inductor.
 5. The LED drive circuit of claim 4,wherein the capacitor also provides bulk energy storage.
 6. The LEDdrive circuit of claim 4, further comprising a second capacitor, whereinthe second capacitor is connected to the first LED connection point, theoutput side filter and bulk energy storage device, and the firstinductor and to the circuit input.
 7. The LED drive circuit of claim 4,wherein the circuit provides a regulated current to the LED arrayconnected to the first LED connection point and the second LEDconnection point.
 8. The LED drive circuit of claim 4, wherein thecircuit provides a regulated voltage to the LED array connected to thefirst LED connection point and the second LED connection point.
 9. AnLED drive circuit, comprising: a first LED connection point and a secondLED connection point for connecting an LED array to an output of thedrive circuit; an output side filter and bulk energy storage device,wherein a first connection point of the output side filter and bulkenergy storage device is connected to the first LED connection point, asecond connection point of the output side filter and bulk energystorage device is connected to the second LED connection point, and athird connection point of the output side filter and bulk energy storagedevice is connected to a cathode of a free wheeling diode, wherein theoutput side filter and bulk energy storage device includes a capacitorthat provides bulk energy storage by storing energy from one power linecycle to a next power line cycle and output side filtering; the freewheeling diode, wherein the cathode of the free wheeling diode isconnected to the third connection point of the a second inductor,wherein the second inductor is connected to the second LED connectionpoint and to the capacitor and the cathode of the free wheeling diode,and the capacitor is connected to the first LED connection point and thefirst inductor and to the second inductor and the cathode of the freewheeling diode, and wherein the capacitor and the second inductorprovide output side filter and bulk energy storage device and an anodeof the free wheeling diode is connected to a first inductor and aswitch; the first inductor connected to the anode of the free wheelingdiode and the switch and to the first LED connection point and to thethird connection point of the output side filter and bulk energy storagedevice; the switch connected to the anode of the free wheeling diode andthe first inductor and to a first drive circuit input; and a controldevice for controlling the switch to provide power factor correction;wherein the drive circuit provides both power factor correction and loadregulation using the switch, and wherein the drive circuit is operablefor direct connection to an output of a bridge rectifier, wherein thefirst drive circuit input connects a first output terminal of the bridgerectifier to the switch and a second drive circuit input connects asecond output terminal of the bridge rectifier to the first loadconnection point, the first connection point of the output side filterand bulk energy storage device, and the first inductor.
 10. The LEDdrive circuit of claim 9, wherein the second inductor also provides bulkenergy storage.
 11. The LED drive circuit of claim 9 further comprisinga second capacitor, wherein the second capacitor is connected to thefirst LED connection point, the output side filter and bulk energystorage device and the first inductor and to the circuit input.
 12. TheLED drive circuit of claim 9, wherein the circuit provides a regulatedcurrent to the LED array connected to the first LED connection point andthe second LED connection point.
 13. The LED drive circuit of claim 9,wherein the circuit provides a regulated voltage to the LED arrayconnected to the first LED connection point and the second LEDconnection point.